Structures related to the present invention are described in WO 98/20933 and the following U.S. Pat. Nos. 5,300,107; 4,796,643; 5,179,962; 4,402,329; 4,432,377; 4,269,198; 4,479,500; 4,945,922; 4,716,888; 4,437,475; 4,917,106; 5,336,253; 5,303,740; 5,074,313; 5,090,422; 5,423,881; 4,721,118; 4,662,382; 4,585,013; 4,582,069; 4,506,679; 4,497,326; 4,467,817; 4,301,815; 4,444,206; 4,409,994; 4,258,724; 3,943,936; 3,971,364; 3,902,501; 5,439,485; 4,488,561; 4,360,031; 4,443,289; 4,988,347; 4,454,888; 4,643,201; 5,016,646; 5,044,375; 5,231,996; 5,405,374; 4,393,883; 4,332,259; 4,402,328; 4,156,429; 4,590,950; 4,458,677; 4,236,529; 4,913,164; 4,841,971; 4,722,353; 4,289,144; 5,476,499; 5,476,500; 5,476,502; 5,425,756; 5,324,327; 5,261,418; 5,257,634; 5,238,007; 5,738,220; 5,713,945; 5,683,447; 5,578,068; 5,571,157;5,545,206; 5,562,723; 5,423,881; and 5,645,580. The contents of each of these U.S. patents is hereby incorporated by reference into this application.
Practically all of the solutions described in the documents cited above provide for the anchoring structure to be produced in the form of a body from which one or more projecting anchoring elements usually called "barbs" (or "tines" in current terminology) extend in a configuration generally comparable to that of an anchor. With a certain degree of simplification, but without departing very much from reality, the configurations of the tines in the above-mentioned documents can be divided substantially into two basic types: (1) the type which provides for the tines to be produced in the form of small bars which are generally cylindrical throughout their length between the proximal region connected to the body of the structure and the distal end (see, for example, U.S. Pat. No. 4,269,198); and (2) the type in which the tines have a generally flattened configuration, possibly with dimensions which decrease gradually from the proximal region (of substantially elongate shape) connected to the body of the structure, towards the distal end. An example of this second type of configuration is described in U.S. Pat. No. 4,945,922. This configuration provides for the use of tines of flattened shape which have a slightly arcuate profile in a generally semi-cylindrical configuration so that the tines can fit better against the wall, which is usually cylindrical, of the body of the anchoring structure when they are folded to the position for the insertion of the electrode towards the implantation site.
In the configuration described in U.S. Pat. No. 4,945,922, the proximal regions of the tines extend along a path substantially aligned with the direction of the planes transverse the principal axis of the body of the anchoring structure. In contrast, in the configurations described in U.S. Pat. Nos. 4,721,118, 4,585,013, and 4,467,817, this proximal region extends along a path substantially aligned with one of the generatrices of the cylindrical body of the structure and hence in a direction parallel to the principal longitudinal axis of the body. This configuration (see, for example, FIG. 4 of U.S. Pat. No. 4,721,118) enables the tines to be brought to a position in which they are wrapped around and close to the body of the anchoring structure when it is confined inside a sheath used for positioning it at the implantation site by catheterization. To adopt the terminology which is conventional in the field of the propellers (helices) to which reference will be made below, the configuration described in U.S. Pat. No. 4,945,922 may be seen as a configuration in which the tines of flattened profile have a keying angle of 0.degree.. On the other hand, in the configuration described in U.S. Pat. No. 4,721,118, the tines in question have a keying angle of 90.degree..
Tines of the types referred to above have some intrinsic disadvantages, even when they are used in combination. For example, tines with a bar-like, typically circular profile tend to be too inflexible in the proximal region connected to the body of the anchoring structure. Moreover, when they are folded close to the body of the anchoring structure in the insertion position, these tines tend to project quite significantly relative to the outline of the body of the restraining structure.
Flattened tines with "zero" keying angles can be made to fit quite closely against the body of the restraining structure at the insertion stage. However, their small cross-section in the proximal region means that the tines often have inadequate behavior during the resilient opening-out stage after positioning at the implantation site. Moreover, the low resistance of the proximal region to bending exposes the tines to the risk that even a slight stress applied to the electrode in the direction away from the implantation site causes the tines to turn over from the generally anchor or arrow-like (harpoon-like) configuration which can ensure firm anchorage of the electrode at the implantation site.
Tines with "90.degree." keying angles have the undoubted advantage of rendering independent the flexural characteristics of the proximal regions of the tines which come into play, respectively, when the tines are wrapped around the body of the anchoring structure, and when they are unfolded from the body in question, projecting radially like fins relative to the anchoring structure. In the first situation, the proximal regions of the tines are in fact subjected to bending stress relative to their smallest dimension, thus showing a high degree of flexibility. In the second situation, the bending stress acts in the direction in which the extent of the proximal regions of the tines is greatest so that they show much greater strength and stiffness.
However, even this latter solution is not free of problems. It in fact imposes limitations due to the number and radial extent of the tines which can be arranged on the body of the anchoring structure in the same region of its axial extent. This is because it is necessary to prevent the tines from coming close together and interfering with one another while they are being wrapped around and close to the body of the anchoring structure. This is disadvantageous both because of a possible increase in the radial dimensions of the unit due to the superimposition of the tines, and because of possible problems of interference during the unfolding stage. In this connection, it should be noted that the unit formed by the tines and by the body of the anchoring structure is usually a one-piece elastomeric component which has the appearance of a bush from which the tines extend.
There is, moreover, a tendency to reject solutions which provide for the use of tines which are offset relative to one another along the axis of the body of the anchoring structure, since it is usually preferred to be able to fit at least four tines uniformly distributed angularly on the same axial portion of the body.
The object of the present invention is to provide an anchoring structure of the type specified above in which the above-mentioned problems are finally overcome. According to the present invention, this object is achieved by means of an anchoring structure having the specific characteristics described in this specification and recited in the claims.